Method of evaluating the corrosion rates of metals

ABSTRACT

A test piece of metal with a given area S is disposed with a reference electrode in test solution. The open circuit potential E cor , i.e. the corrosion potential, of the test piece is measured as the reference electrode potential. A given amount of charges q from a capacitor is instantaneously fed through the reference electrode to the electrical double layer of the test piece. When that a given amount of charge q is stored in the electrical double layer, the absolute value of polarization potential of the test piece sharply increases. Then, the absolute value of the polarization potential gradually decreases due to the corrosion reaction. The polarization potential variation is recorded referred to the reference electrode as a polarization potential (η t ) - time (t) curve by a potential recorder with an extremely high input impedance. The measurement result of the polarization potential (η t ) - time (t) curve may be theoretically expressed by the equation log η t  = -t/(C D  R p ) + log η 0 . Therefore, the initial polarization potential η 0  may be obtained by extrapolating the measurement result to the initial time t=0. The differential capacitance of the double layer C D  can readily be calculated by the equation q/S = Δq = C D  η 0 , and thus the polarization resistance R p  is obtained from the measurement result. Since the polarization resistance R p  is inversely proportional to the corrosion rate, the corrosion rate may be evaluated. The corrosion rate also is obtained from the polarization resistance R p  by using a theoretical equation.

BACKGROUND OF THE INVENTION

The present invention relates to a coulostatic method of evaluating thecorrosion rates of metal and a measuring apparatus used for same.

The weight loss method has been long known for evaluating the corrosionrates of metal. This direct method is able to measure the corrosion rateof a metal surely and accurately, but it requires a long time to obtainthe measurement result and fails to obtain the corrosion rate changewith respect to time.

Recently, the polarization resistance method has been used forelectrochemical evaluation of the corrosion rates of metal. Thepolarization resistance method is disclosed in "J. Electrochem. Soc.,104,56 (1957)" by M. Stern & A. Geary, "Corrosion, 14, 440t (1958)" byM. Stern, and "Proc. Am. Soc. Testing Materials, 59, 1280 (1959)" by M.Stern & E. Weisert. This method is based on the fact that corrosion of ametal involves an electrode reaction where metal ions are dissolved fromthe metal surface and the rate of the electrode reaction relates to thevalue of the current flowing in a corrosion reaction. In this method, ametal test piece having a corrosion potential E_(corr) in a testsolution is used as a working electrode. A constant current (I) is thenmade to flow from the test metal piece to a counter electrode. Underthis condition, the potential E_(mes) is measured. From the corrosionpotential E_(corr) and the measured potential E_(mes), the change of thepotential η is calculated by 72 = E_(mes) - E_(corr), and then thepolarization resistance R_(p) is obtained by the equation (1)

    η = I·R.sub.p                                 ( 1)

The corrosion rate is obtained from the relation that the polarizationresistance R_(p) calculated from the equation (1) in inverselyproportional to the corrosion rate.

The polarization resistance method using constant current is rapid forobtaining the corrosion rate, compared to the weight loss method, but itstill suffers from the following problems. A relatively long time mustbe taken until the potential E_(mes) of the metal piece reaches aconstant value, i.e. the potential η reaches a constant value. A longmeasurement time permits a continuous current flow over the surface ofthe metal piece, resulting in change of the surface condition of themetal piece. This can cause an experimental errors. In the case that asolution has a large solution resistance, its large ohmic drop givesrise to measurement errors. These errors must be compensated through acomplex measuring operation and calculation.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a methodof evaluating the corrosion rate of a metal in which the polarizationresistance of a metal test piece is rapidly and accurately obtained andthe corrosion rate is calculated from the obtained polarizationresistance.

Another object of the present invention is to provide an method ofevaluating the corrosion rate of a metal in which the corrosion rate isaccurately obtained without any correction for the resistance of thetest solution.

Still another object of the present invention is to provide a measuringapparatus for accurately and rapidly measuring the variation ofpolarization potential of a metal with respect to time.

In one preferred form of the present invention, a method of evaluatingthe corrosion rate of a metal comprises feeding a given amount of chargeq to the electrical double layer of a metal test piece having a givenarea S and disposed in corrosion solution; measuring the value ofpolarization potential η_(t) of said metal test piece, of which thepotential sharply increases due to the application of the given amountof charge and gradually decays due to a corrosion reaction and returnsto the corrosion potential, via a reference electrode disposed alongwith said metal test piece in a corrosion solution, said polarizationpotential being measured as a function of time t; and calculating theinitial polarization potential η₀ at time t = 0 from said polarizationpotential η_(t) measured as the function of time t, and deriving thepolarization resistance R_(p) inversely proportional to the corrosionrate from said initial polarization potential η₀ and the charge q fed tosaid metal test piece and the measured η_(t) - t relationship; wherebythe corrosion rate of the metal is evaluated with reference to saidpolarization resistance R_(p).

In another preferred form of the present invention, a measuringapparatus used for the method for evaluating the corrosion rates ofmetal comprises a corrosion solution: a cell containing said testsolution therein; a metal test piece having a given area immersed in thecorrosion solution in said cell, the corrosion rate of which is to beevaluated; a reference electrode disposed along with said metal piece insaid corrosion solution; means for instantaneously feeding a givenamount of charge to said test metal piece through said referenceelectrode; and recording means for recording the potential of said metaltest piece versus said reference electrode as a function of time.

Other objects and features of the present invention will be apparentupon careful consideration of the following description when taken inconnection with the accompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates in block form a measuring apparatus used for a methodfor evaluating the corrosion rate of a metal according to an embodimentof the invention;

Fig. 2 illustrates the equivalent circuit of the corrosion reactionoccurring in the measuring cell shown in FIG. 1;

FIG. 3 is a circuit diagram of the measuring apparatus shown in FIG. 1;

FIG. 4 shows a graph illustrating the relation of the polarizationpotential η_(t) versus time;

FIG. 5 is a graph illustrating the relationship between the logarithm ofthe polarization potential log η_(t) and time t;

FIG. 6 illustrates in block form a measuring apparatus used for a methodof evaluating the corrosion rate of a metal according to anotherembodiment of the present invention;

FIG. 7 shows a circuit diagram of the measuring apparatus shown in FIG.6;

FIG. 8 illustrates in block form a measuring apparatus used for a methodof evaluating the corrosion rate of a metal according to still anotherembodiment of the present invention; and

FIG. 9 shows a circuit diagram of the measuring apparatus shown in FIG.8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method of evaluating the corrosion rate of a metal is an applicationof the coulostatic method. In the evaluation method, the corrosion rateis analyzed on the basis of the measurement results obtained by themeasuring apparatus shown in FIG. 1. The coulostatic method proposed byP. Delahay is a measuring method concerning electrode reactions, whichis discussed in detail in "J. Phys. Chem, 66, 2204 (1962)" by P. Delahayand "Ibid, 66, 2208 (1962)" by P. Delahay and A. Aramata. In short, thecoulostatic method utilizes the interface between electrode and solutionas serving as a sort of leaky capacitor. In this method, the electricaldouble layer at the interface is instantaneously charged with a givenamount of charges. The electrode reaction occurring due to the chargingof the double layer is recorded as the variation of electrode potentialversus time. The information about the rate of the electrode reaction iscalculated from the recorded data.

The present inventors have invented a method of evaluating the corrosionrate of a metal, considering the fact that corrosion is a sort ofelectrode reaction and that the time variation of a potential has aclose relation to the corrosion rate.

Referring now to FIG. 1, there is shown an embodiment of a measuringapparatus of the invention for obtaining the variation of thepolarization potential η_(t) with respect to time. The measuringapparatus includes a metal test piece 2 with a given area S serving as aworking electrode, such as mild steels, stainless steels, other metalsand the like; a reference electrode 4 disposed facing the test metalpiece 2; and a cell 6 filled with test solution such as water in whichthe test metal piece 2 and the reference electrode 4 are disposed. Theworking electrode 2 and the reference electrode 4 are connected to apulse generator 8 and a high impedance potential recorder 10,respectively. In the measuring apparatus shown in FIG. 1, a given amountof charge in the form of pulse q from the pulse generator 8 isinstantaneously applied to the electrical double layer of the metalpiece 2, through the reference electrode 4. Variation of the potentialη_(t) of the test piece 2 as it is charged is recorded versus thereference electrode as a function of time by the potential recorder 10.A sufficiently short period is preferable for the pulse width of thepulse generated by the pulse generator 8, e.g. several μs to several ms.Preferably, the charge q of the pulse applied to the test piece 2 is ofsuch an extent that the potential of the metal piece 2 changes only byseveral mV. The potential recorder 10 has an extremely high inputimpedance. As a result the current flowing from the metal piece 2 to therecorder 10 via the reference electrode 4 is negligibly small. Thismeans that the polarization potential of the metal piece 2 is measuredsubstantially through an open circuit. The potential decay η_(t) of thetest piece 2 arises from the corrosion reaction of test piece 2progressing and thereby consuming the charge q charged therein. In thismanner, the variation of the potential η_(t) due to the corrosionreaction may be accurately measured.

The explanation to follow is for the method of evaluating the corrosionrate of a metal on the basis of the measured values provided by theapparatus shown in FIG. 1. The metal test piece 2 in the test solutionis at the open-circuit potential, i.e. the corrosion potential E_(corr).When the electrical double layer of the metal piece 2 is instantaneouslycharged with charge q and the absolute value of the potential of themetal piece 2 reaches the maximum E_(mes), the initial polarizationpotential η₀ of the test metal piece 2 is given by the followingequation

    η.sub.0 = E.sub.mes - E.sub.corr                       (2)

As will be seen later, the initial polarization potential η₀ can not bemeasured directly by the potential recorder 10. The reason for this isthat the ohmic drop of the solution resistance R_(s) gives rise toincorrect measurement. The variation of polarization potential η_(t)with time measured by the recorder 10 is theoretically given by

    η.sub.t = η.sub.0 exp (-t/C.sub.D R.sub.p)         (3)

The derivation of the general equation (3) will be referred to later.

The equation (3) may be rewritten into a logarithmic equation

    logη.sub.t - logη.sub.0 = -t/C.sub.D R.sub.p       (4)

where C_(D) is the differential capacitance of the test piece 2 andR_(p) is the polarization resistance of the test piece 2. Both of thevalues are per unit area. The differential capacitance C_(D) may beexpressed as

    C.sub.D = Δq/η.sub.0                             (5)

where Δq = q/S and S is the area of the metal piece 2. The differentialcapacitance C_(D) of the test piece varies with the potential valuethereof, but it is considered to be substantially constant within asmall voltage domain. Δq is charge density.

The polarization resistance R_(p) may be calculated from the datameasured by recorder 10 and equations (4) and (5). Equation (4)describes a rectilinear line on a semi-logarithmic graph. Accordingly,the initial potential η_(O) may be obtained if the measured values ofN_(t) ust are plotted on a semi-logarithmic graph and the plottedrectilinear line is extrapolated to the initial time t = 0. Thedifferential capacitance C_(D) is obtained by substituting the initialpolarization potential η_(O) into the equation (5). The polarizationresistance R_(p) is obtained from the slope of the plotted line and thedifferential capacitance C_(D). Generally, the polarization resistanceR_(p) is inversely proportional to the corrosion rate V. As seen fromequations (6) and (7) to be described later, the corrosion rate V canreadily be calculated from the polarization resistance R_(p). By usingonly the resistance R_(p), therefore, it is possible to decide whetherthe test metal piece is resistive to corrosion in a certain testsolution or not.

The corrosion reaction of the metal piece 2 may be electricallyexpressed by the equivalent circuit shown in FIG. 2. The solutionresistance R_(s) serves as a resistor for the applied voltage when thedifferential capacitance C_(D) is charged through the referenceelectrode 4. In the measurement of the polarization resistance R_(p),the polarization potential η_(t) is measured at substantially opencircuit, and therefore the measured value undergoes no influence due tothe above mentioned solution resistance R_(s). Even if the measuredvalue should undergo a slight influence due to the solution resistanceR_(s), the initial polarization value is obtained through extrapolation.Therefore, the obtained initial polarization value η_(o) is veryaccurate, it undergoes no influence by the solution resistance R_(s).From comparison of the equivalent circuit of FIG. 2 and the equation(3), it will be understood that the corrosion reaction of a metal may beelectrically measured as a transient phenomenon in a closed circuitconsisting of the capacitor C_(D) and the resistor R_(p), that is, thecorrosion reaction may be expressed as a phenomena where the chargedensity Δq of the charged capacitor C_(D) is consumed in the resistorR_(p).

The method will be described for obtaining the corrosion rate of thetest piece 2 from the polarization resistance R_(p).

Generally, the corrosion rate V is expressed

    V = (M/ZF)·I.sub.corr                             (6)

where I_(corr) is corrosion current density and given

    I.sub.corr = (K/2.3)/R.sub.p                               (7)

In these equations, K is a constant inherent to the corrosion reaction,M is the atomic weight of the test piece, Z is the valence of thedissolved metal ion, and F is the Faraday constant. As seen fromequations (6) and (7), the corrosion rate V is inversely proportional tothe polarization resistance R_(p) and, if the resistance R_(p) is known,the corrosion rate is easily calculated.

The theoretical derivation of the equations (3) and (4) will be givenfor a better understanding of the above-mentioned equations.

The "Stern-Geary" equation disclosed in the above-mentioned literaturesis expressed by the equation (8)

    I.sub.t = 2.3·I.sub.corr ·{(β.sub.a + β.sub.c)/β.sub.a β.sub.c }·η.sub.t (8)

where β_(a) = 2.3 RT/(n₊ ·α₊ ·F) and β_(c) = 2.3 RT/(n₋ ·α₋ ·F). I_(t)is the faradaic current at time t, α+ and α- the transfer coefficient ofthe anodic and cathodic reactions, n+ and n- the number of electronsinvolved in the anodic and cathodic reactions I_(corr) is the corrosioncurrent density, R, T and F the gas constant, absolute temperature andfaradaic constant, respectively. We can write Δq_(t), the amount of thecharge density which is consumed from time O to time t by the corrosionreaction, as follows

    Δq.sub.t = C.sub.D (η.sub.O - η.sub.t)       (9)

The charge density Δq_(t) is also expressed by the equation (8) ##EQU1##From the equations (8) and (9), the following differential equation isobtained ##EQU2## Solving the equation (11) under the initial conditionthat η_(t) = η_(O) at t = O, we can derive the following equation

    η.sub.t = η.sub.O exp {-2.3 I.sub.corr ·t/(C.sub.D K)}(12)

where K = β_(a) β_(c) /(β_(a) + β_(c)) and β_(a) and β_(c) are Tafelslopes of anodic and cathodic reaction respectively. As seen from theequation (7), K/2.3·I_(corr) may be replaced by R_(p). Using theresistance R_(p) in place of K/2.3·I_(corr) leads to the equation (3)mentioned above.

    η.sub.t = η.sub.O exp {-t/(C.sub.D ·R.sub.p)}(3)

From the foregoing description, the polarization resistance R_(p) iscalculated from the slope of the plotted line and the differentialcapacitance C_(D) which is calculated from the charge density Δq and theinitial polarization potential η_(O) obtained by extrapolation and thecorrosion rate V is calculated from polarization resistance R_(p) soobtained.

Turning now to FIG. 3, there is shown a measuring apparatus for themethod of evaluating the corrosion rate of a metal according to theinvention. In the figure, the pulse generator 8 connected to the testmetal piece 2 and the reference electrode 4 is comprised of a powersource 12, a circuit 14 including a group of capacitors 14-1 to 14-4 forstoring charge fed from the power source 12, and a relay 16 permittingthe charge in the capacitor 14 to instantaneously discharge into theelectric double layer of the test piece 2. The power source 12 isconnected to a variable resistor 22 through a polarity reversal switch18 and a power switch 20. The relay 16 is provided with a trigger switch24, a normally closed contact 26 connected to a movable contact of thevariable resistor 22, and a normally open contact 28 connected to themetal piece 2. A movable contact 30 of the relay 16 is connected to thefixed terminal of the variable resistor 22 through a rotary switch 32and the capacitor 14, and to the reference electrode 4. The rotaryswitch 32 is used for the respective capacitors 14-1 to 14-4 withdifferent capacitances C₁₄₋₁ to C₁₄₋₄ for selecting a desired one ofthem. The series circuit of the capacitor 14 and the rotary switch 32 isconnected in parallel with the series circuit of switch 34 and voltmeter36. It is necessary that the capacitance C₁₄₋₁ and C_(14-r) of thecapacitors 14-1 to 14-4 be not more than about 1/100, a small valuesufficiently smaller than the capacitance S.C_(D) of the test piece 2 asrepresented by the product of the differential capacitance C_(D) and thearea S of the test piece 2.

A potential measuring system for the working electrode 2, i.e. the metalpiece, is comprised of an operational amplifier 38 as a voltagefollower, the potential recorder 40, and a bias means 42. These areconnected in series between the working electrode 2 and the referenceelectrode 4. The bias means 42 is comprised of a power source 44, apolarity reversal switch 46, an input switch 48, and a variable resistor49. The movable contact of the variable resistor 49 is connected to thetest metal piece 2 and the fixed terminal of the variable resistor 49 isconnected to the potential recorder 40. The bias means 42 serves to biasthe signal fed from the cell 6 so that the recorder 40 records thepotential in a given measuring range. In other words, it is used fordeleting the open circuit potential E_(corr) of the metal piece 2 fromthe output signal.

In operation, the area S of the metal piece 2 in corrosion solution ismeasured and the open circuit potential of the metal piece 2 is recordedby the potential recorder 40. Then, the rotary switch 32 is rotated toselect a desired capacitor 14-2, for example, and the power switch 20 isclosed so that the capacitor 14-2 is charged through the path of thevariable resistor 22, the relay 16, and the rotary switch 32. The amountof charge stored in the capacitor 14-2 can be readily calculated byturning on switch 34 to obtain the potential difference of the capacitor14-2 across voltmeter 36.

Following this, when the start switch 24 is turned on, the relay 16 isoperated so that the movable contact 30 is turned from the normallyclosed contact 26 to the normally open contact 28. At this time, chargestored in the capacitor 14-2 is instantaneously discharged into thedouble layer of the metal piece 2 through the reference electrode 4.When the start switch is opened, the movable contact 30 is immediatelyreturned to the normally closed contact 26.

The charge q stored in the electrical double layer of the metal piece 2is gradually consumed by the corrosion reaction of the metal piece 2 inthe cell 6. Because the value of the capacitance C₁₄₋₂ of the capacitor14-2 is selected to be not more than about 1/100 of the value of thecapacitance S.C_(D) of the test piece 2, most of the charge q containedin the capacitor 14-2 is instantly transferred to the test piece.

The potential E_(t) of the test piece 2 gradually decays with time. Thepotential signal E_(t) of the metal piece 2 is amplified by theoperational amplifier 38 and recorded by the recorder 40. Thepolarization potential η_(t) is calculated from the relation η_(t) =E_(t) - E_(corr) and the variation thereof with time is recorded asshown in FIG. 4. The polarization potential η_(t) is directly recordedby the recorder 10 since the bias potential E_(corr) is applied to themetal piece 2. The measured values of the polarization potential η_(t)are linearly plotted on the logarithmic graph as shown in FIG. 5.Therefore, the initial polarization potential η₀ may easily be obtainedby extrapolation of such a graph to t = 0. By using the initialpolarization potential η₀ and the charge density Δq = q/S and theequations (4) and (5), the differential capacitance C_(D) and thepolarization resistance R_(p) are calculated. From the equations (6) and(7), the corrosion rate V is obtained.

Referring now to FIGS. 6 and 7, there is shown another embodiment of themeasuring apparatus for the method of evaluating the corrosion rate of ametal. In the figures, like reference numerals are used for designatinglike or equivalent portions in FIGS. 1 and 3.

As shown, a counter electrode 50 is provided in the cell of themeasuring apparatus of this example. The additional electrode 50 is usedfor charging the electrical double layer of the test piece 2. In theprevious example, the reference electrode 4 includes the function of thecounter electrode 50 of the instant case. The reference electrode 52 isused merely as a reference electrode for measuring the potential of thetest piece 2.

Because of the difference in electrode disposition, the circuit of FIG.7 differs from that of FIG. 3, as a matter of course. Since the metalpiece 2 is not charged through the reference electrode 52, the electrode52 is not connected to the circuit 14. Because of the counter electrode50, the bias means 54 is connected to the circuit 14. In the case whenthe test piece 2 and the counter electrode 50 have different naturalpotentials, the test piece 2, the counter electrode 50 and the testsolution constitute a battery. Thus a potential difference occursbetween the test piece 2 and the counter electrode 50. As a result, theelectrical double layer of the test piece 2 may not be chargedcompletely. To prevent such an insufficient charging, bias means 54 isarranged in the circuit to make equal the potentials of the test piece 2and the counter electrode 50. The bias means 54 is comprised of a powersource 56, a polarity exchange switch 58, input switch 60 and a variableresistor 62, the movable contact of which is connected to the counterelectrode 50 and whose fixed terminal is connected to the circuit 14.The measuring apparatus of this example is operated in a manner similarto the previous example, for measuring the polarization potential.

With reference to FIGS. 8 and 9 there will be explained still anothermeasuring apparatus according to the invention. The apparatus shown inFIG. 8 is the same as the apparatus of FIG. 1 in basic construction. Itdiffers in that a counter electrode 64 is arranged in the cell 6 inplace of the reference electrode 4 as shown in FIG. 1. Unlike thereference electrode 4 which is made of such a material whose potentialis unchanged if charged by the pulse generator 8, the second metal piece64 is made of the same material as the first metal test piece 2. Forthis reason, a corrosion reaction occurs on the second metal piece 64 aswell as the first metal test piece 2. As a result, the potentialdifference δ between the first and second test pieces changing accordingto the corrosion reaction on both the first metal piece 2 and the secondmetal piece 64 is recorded by the potential recorder 10.

It will now be explained how the polarization resistance R_(p) based onthe recorded potential difference between the first and second testpieces δ may be obtained.

Extrapolation is applied as mentioned above, thereby obtaining aninitial potential difference ι₀ from the recorded value. Then, thedifferential capacitance C_(D) is arrived at by the following equation(13) which resembles equation (5): ##EQU3##

Here, S₁ denotes the area of the first metal test piece 2, and S₂ thearea of the second metal test piece 64. Based on the differentialcapacitance C_(D) and the slope (-1/C_(D) R_(p)) of the line in thesemi-logarithmic graph which provides the extrapolation, thepolarization resistance R_(p) can be obtained.

Equation (13) is formulated in the following way. The charge q betweenthe solution and the electrical double layer of the first test piece 2has the opposite polarity to the charge q between the solution and theelectrical double layer of the second test piece 64. Both charges q areof the same absolute value. Thus, the surface charge density Δq₁ of thefirst piece 2 and the surface charge density Δq₂ of the second piece 64are represented by the following equations (14) and (15), respectively:

    Δq.sub.1 = q/S.sub.1                                 (14)

    Δq.sub.2 = -q/S.sub.2                                (15)

as explained with reference to equation (3), the time-based change ofpolarization potential δ_(1t) due to the corrosion reaction on the firstmetal test piece 2 and the time-based change of polarization potentialδ_(2t) due to the corrosion reaction on the second metal test piece 64are expressed by the following equations (16) and (17), respectively:##EQU4## In equations (16) and (17), δ_(o1) denotes the initialpolarization potential of the first test piece 2, and δ_(o2) the initialpolarization potential of the second test piece 64. In theory, theseinitial polarization potentials can be represented by the followingequations: ##EQU5##

Consequently, equation (16) and (17) are transformed as follows:##EQU6##

Since the difference between δ_(1t) and δ_(2t) is recorded by thepotential recorder 10, the potential difference δrecorded by therecorder 10 is expressed as follows: ##EQU7##

Here, equation (22) is transformed into equation (13): ##EQU8## If S₁and A₂ are equal, that is S₁ = S₂ = S, equation (13) is transformed intoequation (14): ##EQU9##

Thus, differential capacitance C_(D) can easily be obtained by equation(13 ) or (23), just as easily as by equation (5).

The circuit shown in FIG. 9 is substantially the same as that shown inFIG. 3. It differs only in that a short circuit 66 is provided in orderto equalize the open circuit potential of the first metal test piece 2and that of the second metal test piece 64. The short circuit 66 has anormally closed contact 68, which is opened by the relay 16. Through thecontact 68 and a movable contact 70, the first test piece 2 and thesecond test piece 64 are short-circuited with each other while theswitch 24 remains open. When the switch 24 is closed, the movablecontact 30 comes into contact with the normally opened contact 28. Uponcontact between the contacts 28 and 30, the movable contact 70 contactsnormally opened contact 68. As a result, short circuit 66 is opened.

The other constructional and functional aspects of the apparatus of theapparatus shown in FIGS. 8 and 9 are the same as those of theaforementioned embodiments.

Results cutured with the above-described embodiments the measuringapparatuses will be given below, compared to these obtained by theconventional methods.

(1) Measurement by the FIG. 3 measuring apparatus

Metal test piece (working electrode) -- Pure copper plate

Solution -- City water

Open circuit potential (corrosion potential) E_(corr) -- -0.230 V_(vs).SCE

Charge density Δq -- 0.12 μc/cm²

Under this condition, 3.4 mV was the initial polarization potentialobtained by extrapolation of the of the measured date. The differentialcapacitance C_(D) was 35 μF/cm². As as result, the following values wereobtained.

Polarization resistance R_(p) = 213 KΩ·cm²

Corrosion rate V = 0.27 mdd

These values are fairly approximate to the polarization resistance R_(P)= 215 Ω·cm² obtained by the constant current method and the corrosionrate V = 0.266 mdd by the weight loss method. This indicates thatmeasurement by the apparatus of the invention is very accurate.

(2) Measurement by the FIG. 7 measuring apparatus

Test piece -- Mild steel (SS-41)

Solution -- City water

Open circuit potential E_(corr) -- -0.653 V_(vs) ·SCE

Charge density Δq -- 0.3 μ/cm²

Under this condition, the initial polarization potential η_(O) was 4.3mV by extrapolation of the measured data. The differential capacitanceC_(D) was 71 μF/cm². This results in the following muasured values.

Polarization resistance R_(p) -- 2.3 Ω·cm²

Corrosion rate -- 26 mdd

These values are fairly approximate to R_(P) =2.3 Ω·cm² obtained by theconstant current method and the corrosion rate V = 26.5 mdd by theweight loss method. A high accuracy of measurement is proven.

From the foregoing description, it will be seen that the method ofevaluating the corrosion rate of a metal according to the inventionrapidly and accurately promides the polarization resistance R_(p) andthe corrosion rate V without any correction of the measured values.

What we claim is:
 1. A method of evaluating the corrosion rate of metalin corrosive solution comprising:feeding a given amount of charge q tothe electrical double layer of a metal test piece having a given area Sdisposed in corrosive solution, said metal test piece being corroded toemit metal ions in the corrosion solution; measuring the amount ofpolarization potential η_(t) of said metal test piece, the amount ofwhich potential sharply increases due to the application of the givenamount of charge and which gradually decays due to the corrosionreaction back to the open circuit electrode potential measured versus areference electrode disposed with said test metal piece in saidcorrosive solution, said amount of polarization potential being measuredas a function of time; calculating the initial polarization potential η₀at time t = 0 after the cessation of said charge feeding from a plot ofsaid polarization potential η_(t) measured as a function of time, andderiving the polarization resistance R_(p) which is inverselyproportional to the corrosion rate from said initial amount ofpolarization potential η₀ and the charge q fed to said metal test piece;whereby the corrosion rate of the metal is calculated by means of saidpolarization resistance R_(p).
 2. An evaluation method according toclaim 1, in which said initial amount of polarization potential η₀ isobtained by extrapolating the plot of the variation of the amount ofpolarization potential η_(t) as a function of time.
 3. An evaluationmethod according to claim 1, in which said polarization resistance R_(p)is calculated from the differential capacitance C_(D) of the electricaldouble layer of said metal piece which is obtained from the chargedensity Δq = q/S of the charge fed to said metal piece per unit area Sand said initial amount of polarization potential obtained from saidplot of polarization potential variation as a function of time.
 4. Anevaluation method according to claim 3, in which said initial amount ofpolarization potential η₀, said charge density Δq and said differentialcapacitance C_(D) are related by the equation C_(D) = Δq/η₀, and saidpolarization resistance R_(p) is expressed by the slope of the plot ofthe logarithmic function log η_(t) versus time.
 5. An evaluation methodaccording to claim 4, in which said logarithmic function log η_(t) isgiven

    log η.sub.t = -t/R.sub.p C.sub.D + log η.sub.0.


6. An evaluation method according to claim 1, in which said corrosionrate of metal V is given

    V = (m/ZF) · (K/2.3) /R.sub.p

where K is a quotient obtained by dividing the product of the Tafelslopes of the anodic and cathodic reactions by the sum of said Tafelslopes, M is the atomic weight of said test piece, Z is the valence ofthe dissolved metal ion, and F is the Faraday constant.